There are numerous applications in which it is necessary to rapidly turn on and off high-current-capacity power switches to control the power delivered to a load. For example, in an inverter operated in a pulse width modulated (PWM) mode of operation, it may be necessary to cycle power switches carrying a high magnitude of current at a 30 kilohertz switching frequency. Typically, these power switches comprise Darlington-connected bipolar power transistors. At present, the specified storage time of these transistors is on the order of 5 to 7 microseconds. This storage time adversely affects the maximum switching frequency for these transistors and may render them unsuitable for certain applications.
Prior attempts at reducing the turn-off time of Darlington power switches have relied upon the application of a reverse bias to the base-emitter junction of one or both transistors at the time of turn-off to rapidly deplete excess carriers therein. For example, Seager U.S. Pat. No. 3,697,783 discloses transistor switching circuitry that includes a first gating circuit coupled to the base of an input transistor and a second gating circuit coupled to the base of an output transistor. The output transistor is driven on in response to a gating signal by the input transistor and the second gating circuit. During the time the transistors are on, a voltage is built up on a capacitor coupled between the second gating circuit and the base of the output transistor. When the gating signal changes state, a switch is closed to couple the capacitor between the base and emitter electrodes of the output transistor to deplete the number of excess carriers therein so that turn-off can occur in rapid fashion.
Other types of circuits for decreasing the turn-off time of a power switch are disclosed in Akamatsu U.S. Pat. No. 4,389,988, Inami et al U.S. Pat. No. 3,971,961, Carlsen II U.S. Pat. No. 4,234,805 and Baker U.S. Pat. No. 4,210,826 (see FIG. 7 and column 14, line 51 through column 15, line 53).
Prior types of circuits for decreasing the turn-off time of power switches have serious limitations. In the case of circuits like Seager, discussed above, localized "hot spots" or secondary breakdown effects can arise in the base-collector junctions of the transistors due to the rapid depletion of excess carriers. These "hot spots" not only reduce the efficiency of the transistor but also increase the chances of failure thereof. Also, these prior circuits have not achieved the required switching speeds and have not reduced switching losses to an acceptable degree.